1. Field of the Invention
This invention relates generally to bimetal thermostats for use in electric circuits. More specifically, the invention relates to a sealed case thermostat, where, after the thermostat snaps to the open position, the thermostat remains in the open position until the power supplied to the device is turned off or removed.
2. Discussion of the Related Art
Thermostats use a bimetal blade, which is essentially two strips of metal having different coefficients of expansion laminated to each other. With an increase in ambient temperature, the blade will bend away from the side containing the material with the higher coefficient of expansion. In the case of a so-called snap action blade, the blade is molded with a formed midsection so that the bending in response to increased temperature will not be gradual, but will occur suddenly once a threshold temperature is achieved. Thus, the blade will "snap" to its bent position.
For a commercial thermostat, these bimetal blades are incorporated into a case and/or placed adjacent to another contact so that an electrical connection will be either formed or broken as the bimetal blade bends. This type of thermostat has many applications, but has recently been used extensively in the electronics industry, mainly because of the ability to make these thermostats relatively compact.
Recently, new standards have been instituted for some motors, transformers, home appliances, etc., where if the device overheats, the thermostat that is mounted within the device, upon reaching the overheated condition, is now required to snap to the open or off position and thereafter remain in the open position until the power supply is, in some fashion, removed from the device.
U.S. Pat. No. 4,703,298 to Gerson discloses a thermostat that includes ceramic mounting pins 4, 5 that are made of ceramic PTC material. The thermostat is a non-enclosed device and is, therefore, subject to atmospheric conditions. Two metal contact carriers 2, 3 are fastened onto parallel pins 4, 5. Carriers 2 and 3 are selectively moveable relative to each other on pins 4 and 5, so that the thermostat can be adjusted for a specific circuit opening temperature. The thermostat is required to be non-enclosed to permit the carriers 2, 3 to move with respect to the pins 4, 5.
In use, when the bimetal 7 moves to the open position, the temperature is such that the resistance of the PTC material is substantial, so that the current which now flows through the pins generates sufficient heat to keep open the circuit between carriers 2, 3. This current flow, together with the selective resistivity of the pins at this temperature, is sufficient to maintain the bimetal 7 above its reset temperature even though the ambient temperature being monitored by the thermostat may return to its original or normal level. The thermostat thus remains open until it is allowed to reset because it has been disconnected from the circuit, whereby the heat source is removed and the bimetal 7 is permitted to snap to the closed contact position because it is at a temperature below the reset temperature.
U.S. Pat. No. 3,525,914 to Vind discloses a thermo switch that includes a ceramic PTC heat resistor 14 that is mounted between the inner surfaces 19, 20 of the bimetallic strips. Similarly, U.S. Pat. No. 5,309,131 to Hofsass discloses utilizing a PTC resistor 2 mounted between a fixed contact 6 and a moveable contact 7. In all these disclosures, current flows through the PTC resistor regardless of whether the switch is open or closed. The resistance of the PTC resistor increases with increasing ambient temperature. Vind's, Hofsass' and Gerson's switches open when the ambient temperature is above the reset temperature. The PTC resistor in each is designed to generate a sufficient amount of heat so that the temperature is above the reset temperature to maintain the switch open. Thus, the switch of either Vind, Hofsass or Gerson remains open until the load current is removed.
A PTC resistor is designed to have a relatively low resistance when the ambient temperature is below a threshold value. Thus, when the switch is closed, current flows through the PTC resistor, but because the resistance is relatively low, an insufficient amount of heat is generated to increase the temperature at the thermostat above the actuation level. Only when the ambient temperature rises above a predetermined point is the resistance of the PTC sufficient to generate significant heat.
For many uses it is desirable for the thermostat to be enclosed, sealed from the local ambient conditions to ensure the efficient operation of the thermostat. Sealed thermostats can be calibrated or adjusted for a specific circuit opening temperature by distorting the case at a predetermined location as is well known in the art (see, for example, U.S. Pat. Nos. 3,443,259 and 3,223,808, the disclosures of which are hereby incorporated by reference).
Thus, it is an object of the present invention to provide a sealed case hold open thermostat. It is a further object of the present invention to provide a sealed case hold open thermostat that uses a conventional resistor (i.e., the present invention thermostat does not utilize a PTC resistor). In the present invention, significant current only flows through the resistor when the switch is in the open position. Additionally, the amount of heat generated by the resistor is not dependent upon the ambient temperature. There is a need in the art for such a hold open thermostat in which a conventional resistor is electrically connected between a bimetallic blade and a fixed blade or a live case.